In my last lesson students began to think about movement at the microscopic level and they were tasked with starting to design a procedure for an experiment that tests the reaction rate of a calcium carbonate and hydrochloric acid reaction. In this lesson students will use a rubric while working in groups to refine a procedure for this experiment.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. It does this because it sets the stage for student collection of evidence and students will ultimately have to interpret their results.
It aligns to the NGSS Practice of the Scientist of Planning and carrying out investigations by giving students the chance to design an experiment within parameters set by the teacher for safety and materials management. These parameters are important. For safety, most students will be using 1M HCl, but the students who are testing the effect of concentration will also use 3M and 6M. In looking at this, I see the safety parameter of limiting the use of higher concentrations of acid for safety's sake, but also from a materials management perspective. Lower concentrations of acid are less expensive. I have also named the chemical reaction that students will use to demonstrate how reaction rates are influenced; but doing this, I know that the reaction that all students use is safe, and I will not have multiple chemical reactions and reactants permeating the classroom, which could get dangerous or confusing.
The lesson aligns to the NGSS Crosscutting Concept of Stability and Change: Change and rates of change can be quantified and modeled over periods of time because this experiment will help students investigate the chemical change that takes place over periods of time that will vary according to the variable they are manipulating.
In terms of prior knowledge or skills, students will benefit the most from this lesson if they have already spent some time wrestling with creating an experimental design, and in thinking about kinetic molecular theory, even if they do not know the term, as found in this lesson.
There are no special materials needed for this lesson.
I reason that this is a good way to start class because I want to know what students are thinking about their experimental design, and I also want to get them thinking about it.
Mini-lesson: Based on student input, I discover that students do not have an experimental design. I erroneously assumed they were more capable of designing a procedure. When I learn this I decide to explain that I am looking for a few things. On a general level, I am looking for adherence to the rubric—I am looking for materials, their quantities, and how they will be used. I am also looking for specificity; can someone follow this procedure without having to ask clarifying questions? These two criteria are basic to all experimental designs.
At the more specific level, I am looking to see how students plan to capture the mass of the carbon dioxide—are they planning on doing the experiment on an electronic balance? I am also wondering how they plan to manipulate their variable—a procedure that explains how they are heating or cooling their acid, for example, is a procedure that is well thought out.
I then explain that we will work in groups, based on the variable being tested, to identify all of the steps necessary for their procedure. I recommend that they use chart paper or a dry erase board to list all of the ideas that people have, and then number them in order of when the step in the procedure should be completed. I suggest that they have a facilitator whose job it is to ensure that all voices are being heard, and I recommend a scribe—someone who can write quickly and neatly. I note that at the end of the class it is my hope that every student will have a solid first draft of the procedure section of the lab report written and ready to test in the next class.
This instructional choice reflects my desire to see students socially construct their experimental design procedure. I do not want any student to get overwhelmed or fall behind, and I also believe that having several minds bouncing ideas off of one another will create a synergistic effect that will yield more high quality experimental designs.
Student Activity: During this time students divide up by groups based on the variable they were assigned. I walk around the room and listen to the groups. My goal is to answer any questions, to make sure that the group dynamics are healthy, and I might offer a few tips to each group so they know I am invested in their process. As this group re-write video shows, one student is the scribe and she gets ideas from the other members of her group.
Catch and Release Opportunities: While listening to the student conversations I listen for common omissions. I wonder what materials they are planning to use, and if these materials make the most sense. For example, what size beaker are they planning on using?
Stopping class to discuss these types of details is important because I want to get the procedure as tight as I can. I want them to do most of the thinking, but I do not want their lack of lab experience to slow them down. I aim to strike a balance between doing the work for them and having them get frustrated because they keep having to do corrections. Time is also a factor—I could have students do multiple revisions, but I have to balance the need for students to independently develop their procedure with the constraint of time.
Ending class this way allows me to include the three variables in an all-class discussion, and it allows me to really hone in on each group to make sure that they are ready for the next class, when we will do an initial trial of the experiment. I feel like the temperature draft and the surface area draft both show the these groups are ready to do the lab, but I feel that the concentration draft needs revision. That last group understands what they need to do but they have not recorded a procedure that meets the rubric criteria of precise replicable instructions.